1. Field of the Invention
The invention relates to a production line for manufacturing tapered thread nipples of cylindrical graphite sections, through which the graphite sections are moved, cut to length, faced off, provided with storage holes for receiving a thermoplastic binder and with a screw thread, and the storage holes filled with the thermoplastic binder.
2. Description of the Prior Art
Graphite electrodes used in arc furnaces, for instance, for producing electric-furnace steel, consist of several cylindrical graphite sections screwed together. In the operation of the electrodes, the lowest section which forms a base for the arc, is gradually consumed, so that the entire line of electrodes must be lowered proportionally to the consumption rate and a new section must be put in place from time to time at the head of the line. The individual sections are generally screwed together with thread nipples which engage with a recess or socket at the end faces of the sections. The joints of the electrode sections are critical points of the electrode line, since the screw joint can be loosened by the various forces acting on the joint, and also fractures can occur. Loosening results in a higher contact resistance and larger temperature gradients in the electrode line, which eventually also cause a fracture of the connection. Since parts of the electrode line crashing into the hearth of the furnace increase the specific consumption of the electrode and lead to disturbances in the furnace operation, there has been no lack of attempts to improve the strength of the joints in an electrode line and to provide a permanent connection between the parts of an electrode line.
Permanent joints consist frequently of coke bridges which are formed between the surfaces of the socket and the nipple or between the abutting faces of the electrode sections. Such coke bridges are made by providing the nipples or electrode sections with one or several storage spaces which are in connection with the surface of these parts, and the storage spaces are filled with a carbonizible binder which melts or softens at elevated temperature. Preferably, thermosplastic binders are used, for instance bituminous coal tar pitch. If the temperature of the electrode when inserted into an arc furnace, rises above the softening point of the binder, part of the binder flows out of the storage space and fills, at least partially, the empty space between the threads of the joint or the electrode sections. With further rising temperature, the binder is pyrolized and temperature-resistant coke bridges are formed. Greatly different arrangements and designs of the storage spaces have become known, for instance, storage spaces which are worked into the socket or the nipple end face and are filled with pitch, from U.S. Pat. No. 2,510,230, or the storage spaces in the end faces of the electrode sections from U.S. Pat. No. 2,894,776. In these designs the permanent joint extends not at all or only slightly to the screw thread which is included as a cementing surface in an embodiment of U.S. Pat. No. 2,510,230 and according to the teachings of U.S. Pat. No. 2,836,294. The storage spaces arranged in the storage rooms consist here of cylindrical canals or holes which extend essentially transversely to the longitudinal axis of the nipple. Two methods have become known for filling these storage spaces: (1) The storage space is cast full of the melted binder. (2) Preformed bodies consisting of the binder are inserted into the storage space (U.S. Pat. No. 2,836,294). The first-mentioned method is particularly simple, since no forming tools are required and the liquid binder adapts itself to any storage shape. It is merely necessary to heat the binder, prior to pouring, to a temperature of about 30.degree. to 80.degree. C. above its softening point. On the other hand, the vapor developed by hot binders, for instance pitch, which may contain substances harmful to health, are a disadvantage.